Communications systems have evolved rapidly over the last several years to the point where today's communications systems include both audio/visual and data communications. In these systems, signals are sent from one location to another in a variety of ways, such as via transmission lines, airwaves or integrated systems incorporating both. The major benefit of using airwaves is their inherent ability to transfer signals over great distances without the necessity of physical placement of transmission lines. Unfortunately, airwave transmissions are subject to a variety of electro-magnetic interferences. Moreover, these transmissions are less secure in that they can be intercepted by anyone having an appropriate receiver. Still further, where shorter distances are involved, the necessary transceiver systems are often too bulky. It is in these situations that physical cabling is the preferred method of transmission.
The cables for audio/visual and data communications have improved greatly over the years. Early cables were simple, single metallic conductors. Next came multiple metallic conductor cables. More recently, advancing technology has allowed multi-signal transmissions over a single metallic conductor. The resulting cables were able to handle more and more signals, but unfortunately were becoming increasingly bulky.
With the advent of the electronic age and the interest in miniaturization came the need to reduce the size of cabling while increasing cable transmission capacity. Soon a new industry, evolved to meet this need and this industry used optical fibers instead of metallic conductors. To those in the art, it was immediately apparent that fiber optic transmission lines were a great improvement over metallic conductor lines because each fiber could handle a variety of transmissions simultaneously while requiring only a fraction of the space of its metallic conductor equivalent. Because of their capacity-to-space ratios, optical fibers are used today as the cable of choice for thousands of applications. Optical fibers are increasingly the media of choice of EMI/EMP resistant, tamper resistant communications and netting lines. The fiber optic cables can be used above ground, underground and underwater.
Generally, today's fiber optical cables are made up of three parts. They are: 1) the optical fibers; 2) the strength member; and 3) the buffering media. The optical fibers are the actual signal carriers and the strength member is that part of the cable which is designed to bear the load of the other portions of the cable during storage, placement and use. The buffering media is the interface between the fibers and the strength member and is designed to ensure the integrity of the optical fibers during placement and later use.
As those skilled in the art will recognize, modern optical fibers may be multi-mode, single-mode or polarization preserving. Their outside diameters range from 80 microns to 250 microns depending on their type and manufacturer. Each individual fiber is made up of the transmission portion known as the lightwave guide and a protective portion or coating. The lightwave guide is typically less than 20 microns high or wide, and the remainder of the fiber is typically an acrylate protective coating.
As noted earlier, the minute size of the optic fibers facilitates high density cabling. The cabling or packaging of the fibers is critical because the fibers are structurally weak as a result of their material properties and size. The most critical aspect of the mechanical packaging of the fibers is the protection of the fibers from a harsh environment and from the introduction of compressive or tensile strain to the fibers during handling or while in service.
In the past, both these tasks were met by packaging the optical fibers into small diameter, thick-walled plastic tubes. Representative dimensions were a tube diameter of 0.125 inches and a tube wall thickness of 0.030 inches. Each of these tubes housed only 1 to 12 optical fibers. Neither the fibers nor the tubes were suitable load bearing members. Accordingly, these tubes, once filled, were typically wound along a core media or strength member in a helical manner for a multi-fiber, optomechanical cable. The resulting structure was one that was much like those produced in the multi-metallic conductor cable industry. The packaging densities of those designs were limited by the design, characteristics and availability of the plastic tubing. Each tube had to have a specific internal diameter such that, when combined with the winding helix angle, the optical fibers would not bind into the tube's walls during bending, twisting or stretching of the cable. Still further, the tube wall had to be thick enough to prevent buckling or crushing when subjected to cross length handling loads. These loads are present during handling, such as during winding resulting from level wind mechanisms. These loads are also present when the cable is compressed such as when the cable is stepped on or run over. As is clear from the foregoing, the limiting factor of the packaging density was the tube structure and dimensions. The tube structure and dimensions were limited by the load carrying capacity of the strength member. As those skilled in the art will recognize, above ground cable strength members must often be sized by ice accumulation weight. This is so because as the diameter increases, the cable will have a greater top surface area which will accumulate more ice. As the cable diameter increases to accommodate additional optic fiber bundles, the ice accumulation weight increases also. As the ice accumulation weight increases, the use of progressively larger strength members is necessary. This, again, reduces the fiber capacity.
Recent advancements in the protective coatings on individual fibers have resulted in fibers that are virtually self-sufficient in their ability to protect the fiber itself without the need for additional buffering media. Fiber optic cabling technology, on the other hand, has not kept pace with optic fiber technology. Current optical fiber cables are based on the outdated technology described above and presume that the fibers must be packaged in tubes prior to cabling.
In light of the above, it is an object of the present invention to increase the fiber density of an optomechanical transmission cable, while minimizing the outside diameter of the cable. Another object is to provide a structure that obviates the need for packaging the fibers in tubes prior to winding on a core or strength member. Still another object of the present invention is to provide a new fiber buffering system which will allow a significant increase in the packaging density of opto-mechanical cables. Another object is to provide a fiber optic cable providing for easier access to individual fibers allowing branches of cables to be easily and economically created. Yet another object of the present invention is to maximize protection of the optical fibers. Still another object of this invention is to provide a cable that is easy to manufacture and is comparatively cost effective.